1. Field of the Invention
The present invention relates to a circular-polarized-wave converter used in a transmitting-and-receiving device of, for example, a satellite broadcasting system.
2. Description of the Related Art
FIG. 11 is a sectional view of a conventionally known circular-polarized-wave converter. FIG. 12 is a left side view of the circular-polarized-wave converter. The conventional circular-polarized-wave converter shown in these figures comprises a circular cross-section waveguide 10 having a hollow inside portion, and a dielectric plate 11 secured to the inside wall surface of the waveguide 10. The dielectric plate 11 functions as a 90-degree-phase device. The dielectric plate 11 is formed of a dielectric material having a uniform thickness, and has V-shaped cutaway portions 11a at both ends thereof in a longitudinal direction thereof.
In the circular-polarized-wave converter having such a structure, the dielectric plate 11 can convert a circular polarized wave input to the waveguide 10 into a linearly polarized wave and output it, or, in contrast to this, can convert a linearly polarized wave input to the waveguide 10 into a circular polarized wave and output it. In other words, a circular polarized wave is a polarized wave in which a composite vector of two linearly polarized waves that have equal amplitudes and are 90 degrees out of phase rotates, so that, when, for example, a circular polarized wave is input to the inside of the waveguide 10, the phase difference of 90 degrees is eliminated by the dielectric plate 11, as a result of which the phases become the same, thereby making it possible to convert a right-hand circular polarized wave and a left-hand circular polarized wave into vertically polarized waves.
In the circular-polarized-wave converter having the above-described structure, the V-shaped cutaway portions 11a are formed as impedance converting portions at both longitudinal end portions of the dielectric plate 11, so that the reflection components at both ends of the dielectric plate 11 are reduced by the cutaway portions 11a, thereby making it possible to obtain proper input and out impedances. However, since the reflection components cannot be reduced unless tapering angles a (see FIG. 11) of the cutaway portions 11a with respect to a direction of propagation of an electrical wave are made small, lengths L of the impedance converting portions inevitably become large. This has been a serious factor in preventing size reduction of circular-polarized-wave converters. In addition, as shown in electrical field distributions illustrated in FIG. 13, when the dielectric plate 11 is set parallel to an electrical field E1, it is possible to match the impedances in an optimal state with respect to polarized waves in the directions of the electrical field E1 by adjusting the tapering angles a of the cutaway portions 11a. On the other hand, the impedances cannot be matched in an optimal state with respect to polarized waves in the directions of an electrical field E2, resulting in the problem that good low-reflection characteristics cannot be obtained.
Further, in the conventional circular-polarized-wave converter having the above-described structure, since a phase difference is produced by the length of the dielectric plate 11, serving as a 90-degree-phase device, the required length of the dielectric plate 11 is naturally determined. This has been a serious factor in preventing size reduction of circular-polarized-wave converters. Still further, in general, in this type of circular-polarized-wave converter, a required linearly polarized wave/circular polarized wave conversion can be performed in a frequency bandwidth where a phase difference |xcfx86| falls within a range of 90xc2x0xc2x110xc2x0. However, in the above-described conventional structure, the frequency bandwidth where the phase difference falls within the aforementioned range is a relatively narrow frequency bandwidth, so that the conventional circular-polarized-wave converter could not be used as a converter that operates using a wide frequency bandwidth.
Accordingly, in view of the actual state of such a conventional technology, it is an object of the present invention to provide a small suitable circular-polarized-wave converter in which a wide frequency bandwidth can be realized.
To this end, according to a first aspect of the present invention, there is provided a circular-polarized-wave converter comprising a waveguide having a hollow inside portion, and a 90-degree phase device disposed inside the waveguide. In the converter, the 90-degree phase device is a dielectric plate that includes an axial center of the waveguide and that extends in a direction within a plane parallel to the axial center of the waveguide. In addition, stepped portions are formed at both longitudinal end surfaces of the dielectric plate, are positioned on orthogonal axes extending in a widthwise direction and a thickness direction of the dielectric plate, and have two reflecting surfaces that are separated by approximately xc2xc of a wavelength inside the waveguide along a direction of the axial center of the waveguide.
By virtue of this structure, since the phases of an electrical wave reflected by the two reflecting surfaces of the stepped portions are reversed and cancelled, the reflection components at the end portions of the dielectric plate are considerably reduced by the stepped portions, so that the overall length and, thus, the size of the dielectric plate can correspondingly be reduced. In addition, since the impedances can be matched in the optimal state with respect to the polarized waves in both directions of the electrical fields E1 and E2, it is possible to realize good low reflection characteristics.
The stepped portions may be protrusions formed at end surfaces of the dielectric plate, with a protruding amount of each of the protrusions being approximately xc2xc of the wavelength inside the waveguide.
The stepped portions may be recesses formed in end surfaces of the dielectric plate, with a depth of each of the recesses being approximately xc2xc of the wavelength inside the waveguide.
According to a second aspect of the present invention, there is provided a circular-polarized-wave converter comprising a waveguide having a hollow inside portion; a pair of ridges that are provided on an inside wall of the waveguide, and that are 180 degrees apart from each other so as to oppose each other via an axial center of the waveguide; and a dielectric plate that is held by the ridges. In the converter, a length of the dielectric plate and lengths of the ridges in a direction of the axial center of the waveguide are substantially the same.
In the circular-polarized-wave converter having such a structure, a phase difference occurs due to the ridges and the dielectric plate, disposed inside the waveguide, so that compared to circular-polarized-wave converters using a dielectric plate or ridges singly as a 90-degree phase device, the overall length can be considerably reduced. In addition, by combining the positive phase characteristic of the dielectric plate and the negative characteristic of each of the ridges, good converting characteristics can be achieved in a wide bandwidth frequency range.
The dielectric plate may be held by the ridges by fitting a protrusion and a recess. When the dielectric plate is fitted to each of the ridges through a recess or a protrusion, the dielectric plate can be disposed inside the waveguide with high precision, and the stability thereof can be increased.
When the circular-polarized-wave converter of the second aspect of the present invention is used or when the dielectric plate is held by the ridges by fitting a protrusion and a recess, stepped portions having two reflecting surfaces may be formed at both longitudinal end surfaces of the dielectric plate so as to be separated by approximately xc2xc of a wavelength inside the waveguide. In these cases, the phases of an electrical wave reflected at the two reflecting surfaces of the stepped portions are reversed and cancelled. Therefore, the lengths of the impedance converting portions required at the end portions of the dielectric plate can be reduced, so that these structures are preferable from the viewpoint of reducing the size of the circular-polarized-wave converter.